1. Field of the Invention
The present invention relates to an opthalmologic photographing apparatus (also called a “fundus camera”) used for examining the fundus oculi of an eye to be examined and photographing an image of the fundus oculi.
2. Description of the Related Art
In recent years, due to the increasing of the role of computers, the popularizing of video games and the increasing of the ageing population, the demand on opthalmological treatment and examination has been increased. Reflecting such a situation, the treatment and examination efficiencies based on developments of an opthalmologic apparatus in operability are becoming more important.
This trend also exists for opthalmologic photographing apparatus. Improvements has been made not only for hardware, but also for software, in order to achieve, for example, the coordination of various actions and the automation of various settings based on computer program controls.
Such as the software improvements aimed at enhancing efficiency and providing additional functions, the precision of the opthalmologic photographing apparatus as a counter balance must be maintained or even enhanced.
For example, selecting an appropriate aperture size is a key factor to increase the photographing precision. Determining a light volume of observation light and photographing light corresponding to the selected aperture also have a major effect on the photographing precision.
An opthalmologic photographing apparatus dealing with the above issues has therefore been proposed, which features increased operability, efficiency and photographing precision through the coordinated setting of an aperture used for photographing an image of a fundus oculi of an eye to be examined and an illumination light volume. As a selecting mode of the photographing aperture, a resolution priority mode and a depth priority mode may be selected. If the resolution priority mode is selected, an opening area of the aperture will be set from an initial value to a larger preset value, and if the depth priority mode is selected, the opening area will be set from an initial value to a smaller preset value and the light volume will be automatically adjusted in coordination with the altering of the aperture. (See Japanese Patent application Laid open No. 2004-350849)
The above opthalmologic photographing apparatus has been developed based on the following issues:                A CCD camera and a film camera have different resolution and sensitivity, which deems it necessary to change the opening area of the aperture in order to ensure a good precision (a setting of the resolution priority mode and the depth priority mode);        In a case where the opening area of the aperture has been changed, the photographing precise is decreased when the light volume is maintained without changes of the light volume, and the operation efficiency is adversely affected when the light volume is changed manually (an automatic adjustment of the light volume in coordination with changes of the aperture size).        
In other words, the above opthalmologic apparatus has been proposed on the basis of the photographing of the image of the fundus oculi and particularly without considering appropriate the opening area of the aperture for related observation of the fundus oculi.
Accordingly, in the above opthalmologic photographing apparatus, the opening area of the aperture used to observe the fundus oculi will be set to a value (the initial value of the aperture), which is without using a priority mode, that is possible to set an intermediate aperture size between a larger and smaller values. However, when increasing the depth of field in observing the fundus oculi, narrowing the aperture is required, and the following problems may occur:                Focusing is difficult because of the dim observing image;        
Larger depth of field for photographing the image of the fundus oculi leads to difficult focusing;                A narrowed aperture causes increased discomfort to the eye to be examined;        Even when dilated, pupils may still constrict when the luminous flux is high, leading to eclipses and flares.        
The “depth of field” here refers to a range in which a subject is focused back and forth through the subject when being focused with a camera. A wider range means the depth of field is “larger”, while a narrow range means a “smaller” depth of field.